how to use complemented square signal as input for AD633N component

[kenfo1]

Hi all of you
I'm using Multisim 12.0 to do personal research in electronic. In the present lab,  I multiply the  sinusoidal and  square signals using  AD633AN component (U8)  and the circuit  works  fine. But when the  square signal is complemented before being used in   AD633AN  (U1),  the transient analysis fails with a 'time step too small" error.
 Please,  kindly help me to fix this issue.

circuits in  jpeg and Multisim format are attached.

[Tien_P]

 It seems like the digital inverter is causing the convergence problem. I replaced it with two sources and go the circuit to simulate.

[kenfo1]

Hi Tien
your are right. but problem is that I have to use for the two multipliers the square signal and his inverted at the same time.
the circuit does not walk when I use the inverted signal.